Special Report

Worst County for Fall Allergies in Every State

Detailed Findings

Smog forms when pollution from power plants, vehicles, and other sources reacts with ground-level ozone. Rising temperatures and sunlight exposure can speed up this reaction process and contribute to more days of unhealthy smog. In nine of the 10 counties with the highest number of days with unhealthy ozone levels, the temperature rose above 90 degrees fahrenheit for at least 90 days in 2016.

In 16 of the 25 counties with ragweed pollen and the highest number of unhealthy ozone days, the number of 90-plus degree days per year is projected to at least double by 2050. Many counties on this list also receive more exposure to sunlight on an annual basis than the national average. Sunlight also can speed up the smog reaction process.

Dense urban areas tend to be hotter than rural areas and, ultimately, may be worse for individuals with nasal allergies, asthma, and other at-risk groups. In a phenomenon known as the “heat island effect,” building and pavement materials absorb heat less effectively than natural green spaces and result in higher temperatures in built-up areas.

The average temperature in a city with more than 1 million people can be more 5 degrees fahrenheit warmer than its rural surroundings throughout the year, and on some nights, the temperature differential can reach as high as 22 degrees. In the 17 of the 25 counties with the highest number of unhealthy ozone days, the share of residents living in an urban area exceeds the 80.7% national figure.

According to the environmental advocacy group Natural Resources Defense Council, the combination of smog and pollen contributes to more missed days of school and work, more trips to the doctor, higher medical costs, and an increase in premature deaths. In some of the counties with the most unhealthy ozone days, a 25% reduction in the concentration of fine particulate matter in the air is projected to reduce the overall mortality rate by more than 2%, according to data from the Environmental Public Health Tracking Network of the Centers for Disease Control and Prevention.


To identify the worst counties for nasal allergies, 24/7 Wall St. reviewed ozone levels in U.S. counties from the Environmental Protection Agency from 2013 to 2017. Counties were ranked on the average annual number of days when the average concentration of ozone exceeded the 2015 Ozone National Ambient Air Quality Standards (0.070 parts per million). Only monitors that performed at least 75% of scheduled observations in at least four of the five years considered were included in the county average. Additionally, only counties where the presence of common ragweed (Ambrosia artemisifolia) pollen has been reported were included. Ragweed pollen data came from the Center for Invasive Species and Ecosystem Health of the University of Georgia. Ragweed pollen data is unavailable for Alaska and Maryland, which were excluded from our analysis. Population data came from the U.S. Census Bureau’s American Community Survey and is a five-year average for 2011 to 2016. Data on the percentage of the county population living in urban areas also came from the U.S. Census Bureau. Data on the number of days above 90 degrees fahrenheit came from the National Environmental Public Health Tracking Network of the Centers for Disease Control and Prevention and is for 2016. The number of adults age 18 and over with asthma is an estimated figure based on the adult asthma rate from the CDC and five-year population data from the ACS for 2011 to 2016.